Equine abortion and equine rhinopneumonitis are commonly recognized diseases attributed to equine herpesvirus type 1 (EHV-1). Evidence has accumulated indicating that two distinct viruses are involved one predominantly causing abortion, and the other, predominantly causing rhinopneumonitis. The viruses are distinct in their epizootiology, pathogenicity for mice, growth characteristics in cell culture, virus neutralization, restriction endonuclease DNA fingerprints, and viral proteins and by DNA:DNA hybridization the viruses have &lt;20% homology.
The EHV-1 predominantly associated with abortion has been designated subtype 1 or fetal (F) strain, and the EHV-1 predominantly associated with rhinopneumonitis has been designated subtype 2 or respiratory (R) strain. The differences in restriction endonuclease DNA fingerprints led to a proposal that the viruses be designated EHV-1 and EHV-4 for the agents of abortion and of rhinopneumonitis, respectively; this has been agreed to in general. Although the designations EHV-1 and EHV-4 recognize the important molecular differences at the DNA level, they do not remove all of the ambiguities. It is assumed that EHV-1 is acquired as a respiratory tract infective agent and may cause respiratory tract disease; yet, reported isolations of EHV-1 from naturally occurring cases of respiratory tract diseases are few. Conversely, there are five recorded isolations of EHV-4 from aborted fetuses.
Definition of the immunologic relationships between EHV-1 and EHV-4 is central to the control of diseases caused by the two viruses. The present inventor has more clearly determined the immunological relationship between EHV-1 and EHV-4 by in vitro lymphocyte blastogenesis in specific-pathogen-free (SPF) foals, and by virus neutralization, using antisera produced in rabbits and SPF foals.